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The United Nation’s World Cities Report 2020 shows that the global urban population has increased from about 28 percent to 56 percent in-between 1950 to 2020. More than half of the cities worldwide having a population above 1 million are located on coastal plains or deltas and are home to 60 percent of the global urban population. The major issue of urban management is urban vulnerability. To improve the quality of urbanization and sustainable development, assessment of urban vulnerability has a great significance. This study investigates the urban vulnerability assessment in the megacity (population >10 million) of Chao Phraya River Delta, Bangkok, Thailand. Due to the complexity of cities and vulnerability concepts, the present research focuses on different aspects of urban vulnerability. A total of fifteen vulnerability criteria are identified according to the multi-criteria decision analysis (MCDA). Finally, the ordered weighted averaging (OWA) technique is applied to generate vulnerability map of different risk conditions. The sensitivity analysis is applied to strengthen the obtained results. The study reveal that about 25-30 percent people of the megacity are living in the area of high vulnerability zone of the lower Chao Phraya river delta. The overall result reveals that essential remedial measures should be taken in the high vulnerable zones to achieve urban sustainability.

Di-(2-ethylhexyl) phthalate (DEHP) is an extensively used and toxic phthalate plasticizer that is widely reported in marine environments. Degradation of DEHP by bacteria from several environments have been studied, but little is known about marine sediment bacteria that can degrade DEHP and other phthalate plasticizers. Therefore, in this study, we enriched a bacterial consortium C10 that can degrade four phthalate plasticizers of varying alkyl chain lengths (DEHP, dibutyl phthalate, diethyl phthalate, and dimethyl phthalate) from marine sediment. The major bacterial genera in C10 during degradation of the phthalate plasticizers were Glutamicibacter, Ochrobactrum, Pseudomonas, Bacillus, Stenotrophomonas, and Methylophaga. Growth of C10 on DEHP intermediates (mono-ethylhexyl phthalate, 2-ethylhexanol, phthalic acid, and protocatechuic acid) was studied and these intermediates enhanced the Brevibacterium, Ochrobactrum, Achromobacter, Bacillus, Sporosarcina, and Microbacterium populations. Using a network-based approach, we predicted that Bacillus, Stenotrophomonas, and Microbacterium interacted cooperatively and were the main degraders of phthalate plasticizers. Through selective isolation techniques, we obtained twenty isolates belonging to Bacillus, Microbacterium, Sporosarcina, Micrococcus, Ochrobactrum, Stenotrophomonas, Alcaligenes, and Cytobacillus. The best DEHP-degraders were Stenotrophomonas acidaminiphila OR13, Microbacterium esteraromaticum OR16, Sporosarcina sp. OR19, and Cytobacillus firmus OR20 (83.68%, 59.1%, 43.4%, and 40.6% degradation of 100 mg/L DEHP in 8 d), which agrees with the prediction of key degraders. This is the first report of DEHP degradation by all four bacteria and, thus, our findings reveal as yet unknown PAE-degradation capabilities of marine sediment bacteria. This study provides insights into how bacterial communities adapt to degrade or resist the toxicities of different PAEs and demonstrates a simple approach for the prediction and isolation of potential pollutant degraders from complex and dynamic bacterial communities.

One of the persistent environmental problems in the provinces of northern Thailand is severe air pollution during the dry season because of open vegetation burning by farmers for land clearance purpose. Aerosol optical depth and Ångström exponent data from MODIS-Terra satellite indicated that from mid-March to April, 2019, entire Thailand was covered with a high concentration of fine-sized aerosols. Trace metal concentrations of PM2.5 collected from Chiang Rai in northern Thailand and Bangkok in southern Thailand between January and April 2019 were analyzed. Average concentrations of crustal metals such as Al, Ca, and Fe are higher in Chiang Rai compared to that in Bangkok. The Fe/Al ratio in Chiang Rai decreases from 1.65 during the onset of haze to 0.87 during the peak haze approaching a crustal ratio of 0.48. In contrast, Bangkok has higher Na, Mg, and Zn with an average Na/Mg ratio of 6.07 indicative of a sea spray (Na/Mg ∼ 8) origin. Principal component analysis identifies three possible sources in Chiang Rai: (1) crustal dust and biomass burning, (2) industrial source, and (3) refuse incineration mixed with road dust; and for Bangkok (1) natural background, industrial emissions, and coal combustion, (2) traffic emission, and (3) sea spray. The ranges of Pb isotope ratios in the bulk fraction of PM2.5 in Chiang Rai (206Pb/207Pb = 1.1445–1.1657 and 208Pb/207Pb = 2.4244–2.4468) and Bangkok (206Pb/207Pb = 1.1343–1.1685 and 208Pb/207Pb = 2.4138–2.4450) are not significantly different. However, in a time series plot, 206Pb/207Pb ratios in Chiang Rai follow PM2.5 during the peak burning season and correlate well with the Al/Pb (r2 = 0.61) ratios, indicating that at least part of the Pb is derived from crustal dust during peak fire. Using a binary mixing model, the most radiogenic Pb isotopes in Chiang Rai during the peak haze can be explained by ∼5 to 30% mixing of crustal dust with ∼35–40% biomass burning generated aerosols with the background. From the trace metal systematics and Pb isotope ratios, it is evident that (1) during the biomass burning season, trace metals from Chiang Rai are not transported down south to Bangkok and (2) in addition to metals released from biomass burning, the raging fire remobilizes crustal dust that forms an important source of Pb and other trace metals in the Chiang Rai aerosol. Ministry of Education (MOE) Accepted version This research has been supported by a Singapore Ministry of Education (MOE) Tier 1 grant (MOE-NTU_RG125/16-(S)), Department of Science & Technology (DST, Govt of India) (RTF/2019/000052), Centre of Excellence on Hazardous Substance Management and the 90th Anniversary (Ratchadaphiseksomphot Endowment Fund) of Chulalongkorn University Funds. JK was supported by the Science Achievement Scholarship of Thailand (SAST). The authors also thank the research groups of MODIS for providing the data and HYSPLIT for providing software for back-trajectory computations. The authors would also like to thank Mr. Pornsak Bureekul for collecting the sample from Chiang Rai and Ms. Lin Ke for keeping the chemistry lab and instruments in impeccable conditions. Baan Hom Muen Lee Resort in Chiang Rai (http://www.baanhommuenlee.com/) kindly provided us with the pictures of clear day and hazy day in Chiang Rai used in the TOC graphics.

The expression of the abrupt cooling of the 8.2 ka event in the tropics is poorly understood. In this study, we applied lithostratigraphy, loss on ignition, grain size and pollen analyses to sedimentary sequences from Thale Noi, which is located in the west coast of the Gulf of Thailand, in order to reconstruct the regional environmental history during the 8.2 ka event. Our results, based on sediment. This characteristics and pollen records, indicated that the study area was likely to have flooded prior to c. 8300 cal. a BP. The fall in sea-level coincided with a high precipitation, inferred from grain size, between c. 8250 and 8100 cal a BP. The increased precipitation can be attributed to the strengthening of the northeast (winter) monsoonal rains in the western Gulf of Thailand. The timing of the sea-level fall and strengthen stronger winter monsoon is consistent with the abrupt decrease in the temperature records emanating from the northern hemisphere. Subsequently, we observed a sea-level rise from c. 8100 to 7950 cal a BP, which potentially continued till c. 7650 cal. a BP. The possibility of coastal erosion was detected by a hiatus in the sedimentary sequences dated between c. 7950 and 7650 cal a BP. Then, a standstill of the sea-level occurred from c. 7650 to 7100 cal a BP. This transgression is suggested to have caused coastal erosion that resulted in a hiatus in the sedimentation process between c. 7950 and 7650 cal a BP. Correspondingly, fluctuations in the monsoon strength resulted in an overall decreased precipitation from c. 8100 to 7450 cal a BP, although punctuated wet intervals were observed at c. 7950 and 7575 cal a BP.

Fifteen native luminescent bacteria were isolated from the Gulf of Thailand, and their sensitivity for the detection of toxicity of crude oil and its aromatic components was investigated. Of these isolates, Vibrio campbellii strain FS5 was one of the two most highly inhibited bacteria at all crude oil concentrations. This bacterium showed a decrease in luminescence intensity of between 10.7 and 80.2% after a 15-min exposure to 0.0001–10 mg/L of crude oil. The degree of bioluminescence inhibition increased with increasing concentrations of crude oil. The presence of crude oil at all concentrations had negative effects on the log bioluminescence per log number of viable cells after 15- to 105-min exposure. About 10 to 100 times, lower half maximal effective concentration (EC50) values were observed for polycyclic aromatic hydrocarbons (PAHs) than those for benzene, toluene, ethylbenzene, and xylene (BTEX). In the presence of each individual BTEX and PAH, the bioluminescence inhibition increased with increasing exposure time (1–32 h). This indigenous bacterium can be used as a simple and general indicator of oil contamination and its impact on coastal waters as well as for assessing potential toxicity during oil bioremediation.

To improve knowledge of the relationships of human activities with microplastic pollution in the urban estuary in Phuket province, which has a densely populated city on the western coast of Thailand, a total of 463 plastic-like items from 24 sediment samples in the dry and the rainy seasons were identified by micro-Fourier transform infrared spectroscopy. The microplastic abundance ranged in 300–900 and 33–400 items/kg dry weight in the dry and the rainy seasons, respectively, indicating that the estuary is moderately contaminated with microplastics. The most abundant polymer types were rayon and polyester with colored fibers, suggesting that the microplastics deposited in this area originate mainly from washing effluents. Additionally, our findings show that the microplastic distribution is significantly governed by hydrodynamic energy in the estuary. This provides basic information for a better understanding of the fate of microplastics within estuary, and for management actions to address microplastics in urban estuary.

Total mercury (T-Hg) was examined in 8 threadfin bream species (Nemipterus spp.) caught in the Gulf of Thailand (GoT). The T-Hg contents ranged from 11.3 to 374 μg kg

Summary: Acid volatile sulphide (AVS), one of the most reactive phases in sediments, is a crucial link in explaining a dynamic biogeochemical cycle in a marine ecosystem. Research gaps exist in describing the spatial variation of AVS and interconnections with sediment covariates in the Eastern Upper Gulf of Thailand. Measurements of AVS and auxiliary parameters followed the standard protocol. A comparison of ordinary kriging (OK), cokriging (CK), and regression kriging (RK) performance was evaluated based on the mean absolute error (MAE) and root mean square error (RMSE). The concentrations of AVS ranged from 0.003 to 0.349 mg g−1 sediment dry weight. Most parameters contained short range spatial dependency except for oxidation–reduction potential (ORP) and pH. The AVS tended to be both linearly and non-linearly related to ORP and readily oxidisable organic matter (ROM). The RK model, using inputs from the tree-based model, was the most robust of the three kriging methods. It is suggested that nonlinear interactions should be taken into account when predicting AVS concentration, and it is expected that this will further increase the model accuracy. This study helps establish a platform for ecological health and sediment quality guidelines. Keywords: Spatial estimation, Acid volatile sulphide, Sediment, Geostatistical analysis, Gulf of Thailand

The n-alkanes content in the sedimentary sequence from Thale Noi, on the west coast of the Gulf of Thailand, was analyzed and used to reconstruct the past sea-level fluctuations. The results were further compared with those based on sediment characteristics, loss on ignition, grain size distribution, and pollen analysis from the same core to investigate the efficiency of n-alkanes as a paleoenvironmental proxy. The analysis of n-alkanes indicated the likely periods of regression and transgression from c. 8300–7950 cal. a BP and from c 7950–7665 cal. a BP that agrees well with the reconstructed sea-level changes from the sediment characteristics and pollen records. The steady abundance of mangrove pollen taxa in the sediment core points to a standstill in the sea-level from 7650–6870 cal. a BP. However, the results from the n-alkanes analysis further suggested a sea-level fall at c. 7650–7610 cal. a BP and at c. 7400–6870 cal. a BP, and a sea-level rise at c. 7610–7400 cal. a BP. Furthermore, these sea-level changes were intervened by short intervals of regression centered at c.7590 and 7470 cal. a BP.

After global phasing out of leaded gasoline, anthropogenic sources of atmospheric lead (Pb) are dominated by coal combustion emissions, high-temperature metallurgical processes, and vehicle exhausts. Thailand was one of the first countries in Southeast Asia to completely phase out leaded gasoline by the year 1994. This study investigates the sources of atmospheric Pb in Thai aerosols, a quarter-century after phasing out of leaded gasoline using a multiproxy approach, Pb isotopes in conjunction with trace metal composition of PM2.5. Aerosol samples were collected for 1 year from January 2018 to April 2019 from Bangkok and Chonburi to understand the influence of seasonal variation on aerosol chemistry. Bangkok is the only megacity in Thailand and notorious for traffic congestion and Chonburi is a coastal town, 80 km southeast of Bangkok where the inter monsoon winds make landfall. Aerosol Pb concentrations significantly decreased in 25 years from 74 ± 5 ng/m3 in 1994–1995 to 14 ± 13 ng/m3 measured in Bangkok during Northeast (NE) monsoon winds. Concentrations of all the metals are higher in Chonburi compared to Bangkok in the NE monsoon wind, owing to the addition of local emissions from the megacity over the long-range transported metals before the winds reach the coastal town. The Pb isotope ratios have a wide range (206Pb/207Pb range 1.1343–1.1685 and 208Pb/207Pb range 2.4138–2.4450) but the average ratios are not significantly different in Bangkok and Chonburi for all three seasons. However, present day 206Pb/207Pb ratios are more radiogenic than those measured 25 years ago in Bangkok. The Pb isotopes from both locations in all seasons have considerable overlap with unleaded gasoline and diesel used in Southeast Asia indicating automobile exhaust as an important source. Additionally, the NE monsoon winds are influenced by mixing of crustal dust (Al/Pb ratios correlates well with 206Pb/207Pb, r2 = 0.63) with coal combustion emission (strong inter-correlation of Pb, Zn, As and Cd, r2 from 0.72 to 0.94) from China and Vietnam. Pb isotopes of the Southwest (SW) monsoon winds are also influenced by crustal dust and falls on the mixing line between coal combustion and ore processing emission from India in a Pb triple isotope plot. Chemical composition of aerosols in the inter-monsoon winds are similar to SW monsoon winds. After a quarter-century of leaded gasoline banning in Thailand, the atmospheric Pb sources have changed from less radiogenic leaded gasoline exhaust and Chinese ore processing to more radiogenic unleaded gasoline and diesel exhaust and coal combustion.

The high precipitation around Southeast Asia results in abundant freshwater outflow, which transports terrestrial dissolved and particulate material to the world's largest marginal sea, namely, the South China Sea (SCS). To estimate the riverine carbon flux to the SCS, carbonate data from 42 rivers were collected. These results, combined with literature data for 13 rivers, indicate that the concentrations of dissolved inorganic carbon (DIC) are positively correlated with the cation exchange capacity and the bulk density, except for the Low Discharge category. The highest DIC concentration and flux are in the Low and Medium Discharge categories. Negative correlations exist between dissolved organic carbon (DOC) concentrations and the base saturation in the Low and High Discharge categories and between DOC and bulk density in the Low and Medium Discharge categories. However, the correlation between the DOC concentration and the organic carbon content in the soil is only significant in the Medium Discharge category. Once the negative exponential relationships between the particulate inorganic carbon (PIC) or the particulate organic carbon (POC) and the total suspended matter are determined, these can be used to estimate the PIC and POC carbon fluxes. Of the annual riverine carbon flux, 83.0 ± 8.1 Tg C, 40.14 ± 6.11 Tg is DIC, 25.03 ± 4.54 Tg is DOC, 1.83 ± 0.35 Tg is PIC, and 16.03 ± 2.87 Tg is POC. The total discharge amounts to 6.2–10.3% of the global riverine discharge for a sea covering only 1% of the world's ocean surface area and for watersheds covering only 2.2% of the global land mass.

The South China Sea offers an excellent case for studying source-to-sink transport processes of fluvial sediments among the global marginal seas. This study synthesizes existing clay mineralogical and geochemical data from similar to 1500 samples from the seafloor and surrounding rivers, deepwater mooring observation results, and high resolution glacial-cyclic clay mineralogy records from six high-quality sediment cores. Source-to-sink sediment transport from the river mouth to the continental shelf and then to the abyssal basin is investigated at two time scales: modern process and Late Quaternary glacial cycles. The results firstly show the high diversity of clay mineralogical and geochemical compositions in riverbed surface sediments surrounding the South China Sea, e.g., dominant illite and chlorite in Taiwan, overwhelming majority of smectite in Luzon, dominant kaolinite in South China, and similar amounts of the four clay mineral species in Red and Mekong river systems. The formation of these fluvial sediments through chemical weathering in surrounding drainage systems is controlled principally by the East Asian monsoon climate with warm temperature and high precipitation, and subordinately by tectonic activity and specific lithological character. The basin-wide distribution of clay mineral assemblages combined with neodymium and strontium isotopic compositions reflects strong provenance control and differential settling effects. The differential settling of kaolinite in slightly saline and proximal regions relative to smectite in more saline and distal regions is well demonstrated in the South China Sea. Through combining clay mineralogical distributions on the seafloor with observed oceanic current systems, the modern transport pathways can be well established, e.g., in the northern South China Sea, illite and chlorite from Taiwan are mainly carried by contour currents with the strong influence of mesoscale eddies. High-resolution sediment dynamic analysis for the Late Quaternary reveals different sediment transport patterns in the northern, western, and southern South China Sea when the land-sea configuration dramatically changed during glacial conditions. In the north, the terrigenous dispersal is mainly controlled by provenance supply and oceanic current transport. In the west, the clay mineral assemblage reflects variations of the prevailing surface current influenced by monsoon winds. In the south, the clay mineral input indicates intensive chemical weathering during interglacial periods and strengthened physical erosion during glacial periods. The transport of terrigenous sediments since the last glaciation is quantitatively reconstructed through studying two deepwater sediment cores located in the northern South China Sea. The relative contributions from three provenances (South China, Luzon, and Taiwan) highlight their distinct variability over the last 28 ka, implying that the southward shift of the inter-tropical convergence zone (ITCZ) at 16 Ka BP caused an increased sediment contribution from South China, the intensified influence of the Kuroshio Current intrusion controls more efficient westward transport of Luzon sediments, and the stronger deepwater current transports Taiwan-sourced sediments further westward from the last glaciation to the Holocene. (C) 2015 Elsevier B.V. All rights reserved.

This study aims to define numerical sediment quality guideline (SQG) based on equilibrium partitioning approach that represents availability of metal in estuarine sediment. The labile-phase metal was used in calculating the partition coefficient caused by remobilization of this fraction in the sediment-interstitial water system as a result of changing physicochemical condition in sediment. In addition, the metal bound with sulphide fraction was taken into account in calculating the SQG since this fraction is potentially available to interstitial water when oxidation process occurs in anoxic sediment. The SQG values for Chao Phraya estuarine sediment for cadmium, copper, lead and zinc were found in the range of 1.2-3.1, 7-105, 23-86 and 16-125 mg/kg dry weight in

This study presents high-resolution clay mineralogy of Core MASS-III-10, located in the southeastern Andaman Sea. The study aimed to investigate terrigenous sedimentary input from various potential provenances throughout different periods of geological time. The clay mineral assemblages of MASS-III-10 reveal a high amount of smectite at 73% (65-79%), a moderate kaolinite at 12% (10-17%), and a low illite and chlorite content(

In this study, we attempted to use PAHs as a chemical proxy to trace the transport of land-derived materials caused by the tsunami backwash to better understand how it may have affected the distribution of sedimentary deposition throughout the seabed of Khao Lak coastal areas. By analyzing the compositions of sedimentary PAHs in combination with application of the multivariate descriptive statistical techniques, PAHs were proven to be a promising chemical proxy to indicate the tsunami backwash in the study area. Their spatial distribution could indicate that the tsunami backwash plays an important role in transporting anthropogenic PAHs to the nearby coastal area as far as approximately 25 km from the shoreline. In addition, the results from diagnostic PAH isomer ratios suggested that road paving asphalt, originated from heavy erosion by the tsunami wave in front of Pakarang Cape, was among the identified sources of PAHs. Principle Component Analysis (PCA) results provided 2 estimated land-derived sources of PAHs, which were the road dust and oil burning sources. These estimated signature sources clearly support our hypothesis that PAHs were transported from the potential sources on land and deposited into the near-shore seabed during tsunami backwash.

Water scarcity is the main problems of water management and various socio-economic development activities of Thailand. Expert opinion and scientific knowledge are always convenient to be used to solve problems. To some extent, this approach may not be applicable since it is not accepted by local communities. To reduce this conflict, participatory approach may provide local cooperation and assist developing better water management strategies. Pranburi Basin is an important headwater that supports both agricultural land and municipality which includes Hua-Hin, a famous tourist place. The aims of this study were to assess water scarcity and to apply stakeholder participation into decision making processes for water management. Water stress index and criticality ratio were used to assess water scarcity. A Driver-Pressure-State-Impact-Response (DPSIR) framework was used to describe water scarcity and contributing factors. Based on DPSIR analysis, the stakeholders proposed and then prioritized for the most appropriate responses by ordinary weighing technique. The results indicated that the four most preferable measures for this basin include controlling agricultural area, controlling crop types, reforestation and establishing local water user association.

This study retrospectively analyzes the daily results of relative response factors (RRFs) of polycyclic aromatic hydrocarbons detected by gas chromatography–mass spectrometry (GC-MS). Although instrumental routine maintenance can enhance the reliability of measurement, there is no quantitative study to investigate the effects of glass liner contamination, manifold temperature drop, and column degradation on deteriorating sensitivities and stabilities of RRFs. This study demonstrates that by removing the contribution of outliners to the background level, great reductions of RRFs were achieved. Although several factors potentially undermined the analyzer's confidence on data reliability, there were no significant differences on the relative sensitivities of RRFs.

The U-Tapao Canal is the main source of freshwater draining into the outer part of Songkhla Lake, which is the most important estuarine lagoon in Thailand. Songkhla Lake is located in southern Thailand between latitudes 7808’ and 7850’ N and longitudes 100807’ and 100837’ E. Acetic acid (HOAc)-soluble Cu, Fe, Mn, Pb, and Zn and the total concentration of these metals along with Al concentration, organic carbon, carbonate, sand, silt, and clay contents were determined in 4 sediment cores obtained at selected intervals from the mouth of the canal to 12 km upstream. Readily oxidizable organic matter in the cores varies from 1.52% to 7.30% and is generally found to decrease seaward. Total concentrations of Al (61.7–99.0 g kg 1 ; 2.29– 3.67 mol kg 1 ), Cu (12.4–28.2 mg kg 1 ; 195–444 Amol kg 1 ), Fe (25.2–42.0 g kg 1 ; 451–752 mmol kg 1 ), Mn (0.22–0.49 g kg 1 ; 4.0–8.9 mmol kg 1 ), Pb (16.7–43.1 mg kg 1 ; 80.6–208 Amol kg 1 ), and Zn (48.6–122.7 mg kg 1 ; 0.74–1.88 mmol kg 1 ) vary to a certain extent vertically and seaward in the U-Tapao Canal core sediments. These concentrations are at or near natural levels and show no indication of anthropogenic contamination. Overall, the data show that total metal concentrations in the surface and near surface core sediments are enriched in varying degrees relative to Al in the order of ~ZnNMnNPbNFeNCu. Chemical partitioning shows that the enrichment in the surface and near surface sediments is related to the relatively high proportion of the total metal concentrations (MnNZnNFeNCuNPb) that occur in the acetic acid-soluble (nondetrital) fraction, and they generally decrease with depth. Nondetrital Cu, Pb, and Zn likely derive from those metals held in ion exchange positions, certain carbonates, and from easily soluble amorphous compounds of Mn and perhaps those of Fe. Diagenetic processes involving Mn and to a lesser extent, Fe compounds, as well as the vertical changes in the oxidizing/reducing boundaries, appear to be the most important factors controlling the behavior of the metals in these cores. Organic matter and the aluminosilicate minerals, however, appear to be less important carriers of the metals studied. D 2004 Published by Elsevier B.V.

Thai people have always been agricultural-based, although rainwater is not enough to sustain year-round agricultural practice. The first earth-filled dam was built across a valley near Sukhothai over 700 years ago. Now, there are six major multipurposed dam reservoirs, among which the largest is the Sri Nakarind (SN) built across the Kwae Yai River, the main branch of the Mae Klong River in the western part of Thailand. The first multipurposed dam reservoir built on the Chao Phraya (largest) River System is the Bhumibhol Dam on the Ping River. In this paper, the two Mae Klong River Dam Reservoirs and their biology and chemistry are discussed. Also investigated are the historical records of material fluxes to the Gulf of Thailand in down-core sediment profiles and in 40-year bands of coral, Porites lutea, taken from the Upper Gulf of Thailand near Si Chang Island. Sediment cores studied did not show clear decreases in the biogenic silica concentration in the Upper Gulf area after the building of dams. Part of the reasons might be the higher rate of dissolution of silica in tropical waters bioturbation of surface layers or the constant disturbance of the bottom sediment by fishermen using bottom trawls. The record in the coral bands seems to be more promising in reflecting the material flux change due to dam construction. An attempt to detect changes in diatom population with time due to the expected decrease in siliceous material discharge was not possible because there was a change from the use of 70 μm mesh size to 20 μm mesh size resulting in the increase of species and biomass, although the diatom percentage remained above 80% in all estuarine samples.

The rapidly increasing population and developing agroindustrial activities in Thailand exert considerable stress on the aquatic environment. Untreated municipal wastes are discharged directly or indirectly to canals and rivers, causing high BOD5 values and bacterial contamination close to populated areas. Improvements in domestic sewage treatment are likely to be prohibitively expensive in the near term at least. Eutrophication of coastal waters is an emerging problem. By contrast, few problems have been documented from trace metals discharged by industries, and the public health threat from seafood contamination does not appear to be significant. Uses of chlorinated hydrocarbon pesticides are declining sharply, these being replaced by organophosphate compounds which degrade rapidly in aquatic environments. Oil pollution has not been a problem, although fears of a major spill exist.

The coastline of Khao Lak (Thailand) was heavily damaged by the 2004 Indian Ocean tsunami. Onshore tsunami deposits and satellite images, which show large amounts of sediment transported offshore, indicate that the seafloor was impacted by tsunami run-up and backwash. In this study, high-resolution maps of sediment distribution patterns and the geological development of the seafloor are presented. These maps are based on multibeam, side-scan sonar and seismic profiling surveys offshore Khao Lak. Paleoreefs, with associated boulder fields and sandy sediment dominate the inner continental shelf. Patches of fine-grained (silt to fine sand) sediments exist in water depths of less than 15 m. The sediment distribution pattern is stable between 2008 and 2010, apart from small shifts regarding the boundaries of the fine-grained sediment patches. In one sediment core and several grab samples an event layer was documented, situated below a cover of modern sediments which is only a few cm thick. The event-layer is traced down to 18 m water depth. It consists mostly of sand and contains compounds of terrigenous origin. It is interpreted as a 2004 Indian Ocean tsunami deposit. However, over large areas of the study-site, the impact of the tsunami is hardly identifiable by seafloor morphology or sediment distribution.